As electronic equipment becomes smaller and lighter with higher performance, it is necessary to reduce the width of wiring lines and the diameter of the via holes which connect layers in multi-layer printed wiring boards. It is extremely difficult to use mechanical drilling to form holes below about 200 .mu.m diameter, and lasers have been used to make such small holes.
A carbon dioxide laser can form holes at high speed in organic substances, such as epoxy resin and polyimide resin. Such lasers have been widely used in making printed wiring boards. Forming holes in thick copper foil is difficult, however, because the copper foil reflects the laser beam. To solve this problem, as disclosed in Japanese Patent Publication No. 4-3676, a hole is etched through the copper foil having the same diameter as the via hole to be made. Then, the laser beam is used to form the hole through the organic substrate, with the beam diameter being larger than that of the via hole.
In the method of Japanese Patent Publication No. 4-3676, since it is necessary that a hole having the same diameter is etched through the copper foil, followed by irradiating with the laser beam to form the via hole, etching must be done twice, i.e., first, for forming the via hole in the copper foil and, second, for forming the wiring pattern. The repeated etching in such a process results in a serious loss of productivity. This may be compared with the use of conventional mechanical drilling wherein the etching is done only once, that is, for forming the wiring pattern. Furthermore, it is not easy to etch a hole in an outer wiring so as to align the hole with inner pad, because highly accurate alignment is required.
In another process, the faces of an inner insulating layer board which have inner wiring patterns are coated with an insulating resin, and holes are formed in the resin by the laser beam, and then the resin surfaces are directly plated with copper to form an outer copper layer. Only a single layer of copper is deposited. In this process, however, it is necessary to roughen the surfaces of the insulating resin layers formed by applying the insulating resin to obtain acceptable adhesive strength between the plated copper and the insulating resin layer. Roughening of the insulating resin surfaces often cannot provide sufficient adhesive strength between the copper layer and the insulating resin layer.
The present invention solves the above problems of the prior art, and provides a process for making multi-layer printed wiring boards, in which via holes are easily formed with a laser, and adhesion between the outer wiring pattern formed from the plated copper layer and the insulating resin layer between the outer wiring pattern and the inner wiring pattern is improved.